Throngs of demonstrators gathered Monday to witness the raising of the flag of Cuba over the Cuban Embassy, the first time it has flown here since the U.S. broke off diplomatic relations with the communist country 54 years ago. As American and Cuban diplomats stood side by side, three Cuban guards in white uniforms marched out of the building, attached the flag to a recently-constructed flag pole and raised it above Washington's 16th Street, just 2 miles north of the White House, to mark the formal re-opening of the embassy.